1. Field of the Invention
The present invention relates to an overcurrent protection circuit, and more particularly to a circuit protection technique for protecting a voltage generator circuit used as a power supply circuit from overcurrents.
2. Description of the Related Art
In regulators for stabilizing voltages or the like, used as an output transistor is the power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), the IGBT (Insulated Gate Bipolar Transistor), the bipolar power transistor or the like. These transistors are designed to have a sufficient margin between the maximum allowable current and a current flowing during normal operation.
However, even with the transistor designed to have such a sufficient design margin, a short circuit may be developed in the output load circuit. In this case, there was a problem that a large overcurrent exceeding the maximum allowable current would flow through the transistor, thereby affecting the reliability thereof. Furthermore, even when the overcurrent was equal to or below the maximum allowable current of the transistor, it was desired, in some cases, to limit current in order to protect the load circuit connected to the transistor.
In this context, it was a conventional practice to provide the regulator with a protection circuit having a current limiting function in order to protect the transistor from overcurrents or limit the current flowing through the load circuit (see, for example, Japanese Patent Laid-Open Publication No. Hei 5-315852).
However, as pointed out in the aforementioned document, the protection circuit having the current limiting function for sensing the current flowing through the transistor for feedback raised a problem of having adverse effects on the stability of the circuit. That is, a wider frequency band of a feedback loop to improve the capability of tracking variations in output current would introduce instability to the circuit, thereby causing the circuit to readily oscillate. In contrast to this, to assign a higher priority to the stability of the circuit, the gain of the feedback loop and the frequency band could be reduced to make the circuit resistant to oscillation. In this case, however, there raised a problem of being unable to track a sudden change in output current and requiring a long time before the current was actually limited, during which a large current flow would result. That is, the protection circuit having the current limiting function can be said to have a trade-off relation between the stability of the circuit and its sensing speed.